场景：运算中包含null，导致结果为null。
期望：为null时当0计算。
当某字段为null时，赋为0，否则直接运算，结束。
case when
(select SUM(Amount) from rp_traderbillamount WHERE TraderID=126 AND BrandID=1 and Date>='2021-01-01' and Date<='2021-01-31' and BillType=4)
is null then 0
else (select SUM(Amount) from rp_traderbillamount WHERE TraderID=126 AND BrandID=1 and Date>='2021-01-01' and Date<='2021-01-31' and BillType=4)
end
